This invention relates to semiconductor light emitting devices utilizing a gallium-nitride based compound semiconductor adapted for emitting bluish light (from ultraviolet ray to yellow light). More particularly, this invention relates to an improvement in efficiency of outwardly radiating light as well as uniformity of light emission in a light emitting chip for a semiconductor light emitting device which has, on a substrate, semiconductor layers having electrodes respectively formed on a first conductivity-type semiconductor layer on a surface side and on a second conductivity-type semiconductor layer exposed by etching part of the semiconductor layers.
The semiconductor light emitting device for emitting bluish light is structured, for example, by an sapphire insulating substrate and gallium-nitride based compound semiconductor layers formed thereon, as shown in schematic exemplary view in FIG. 10(a). That is, the semiconductor layers includes, for example, an n-type layer (cladding layer) 23 of an n-type GaN epitaxially grown on the sapphire substrate 21, an active layer 24 formed of a material having a bandgap energy lower than that of the cladding layer such as an InGaN-based (having In and Ga variable in ratio thereof) compound semiconductor, and a p-type layer (cladding layer) 25 formed of a p-type GaN. A p-side electrode 28 is provided on a surface of the semiconductor layers, while an n-side electrode 29 is provided on a surface of the n-type layer 23 exposed by etching part of the semiconductor layers. Thus, an LED chip is constituted.
When a forward voltage is applied between the p-side electrode 28 and the n-side electrode 29 of the LED chip thus structured, an electric current flows from the p-side electrode 28 spreading through the p-type layer 25 into the active layer 24, so that it further flows by way of the n-side layer 23 into the n-side electrode 29. In the active layer 24 of the course of the current path, recombination of carriers occur to thereby cause light emission. Incidentally, a current diffusion layer, not shown, of an Ni--Au alloy or the like may be provided on the surface of the p-type layer 25 in order to cause spreading of the electric current throughout the entire p-type layer 25.
Meanwhile, the p-side electrode 28 and the n-side electrode 29 in general are formed in a circular or rectangular shape in plan, as shown in FIGS. 10(b) to 10(c). Accordingly, the p-side electrode 28 and the p-side electrode 29, at their outer peripheries, are distant away from each other so that the electric current path, as viewed in plan, is different in distance from the p-side electrode 28 to the n-side electrode 29 depending upon a location considered, as demonstrated by A and B in the figure.
Moreover, where an current diffusion layer as stated is provided, the current diffusion layer is comparatively low in electric resistance so that the current is liable to readily spread throughout the current diffusion layer. However, an etched form providing the n-side electrode 29 is different from the shape of the n-side electrode 29. As shown in plan views of FIGS. 10(b) to 10(c) (the current diffusion layer is not shown but will be similar in shape to the p-type layer 25), the distance in plan between the current diffusion layer and the n-side electrode 29, in many cases, is differrent as shown by C and D.
If there is difference in distance between the respective electrodes or between the current diffusion layer and an electrode depending upon the current path as stated above, then the electric resistance increases in certain regions where the distance thereof is long. In particular, the gallium-nitride based compound semiconductor, if employed for forming the semiconductor layers, is high in electric resistance as compared to a GaAs-based compound semiconductor so that it is prominent in increase of series resistance in distance considered becomes long. Although the increase in electric resistance for the gallium-nitride based compound semiconductor is prominent for the p-type layer where the dopant is not sufficiently introduced, the n-type layer also has a high resistance increase as compared to the GaAs based compound semiconductor. To this end, if the p-side electrode 28 and the n-side electrode have a low series resistance in a certain plane within the LED chip, i.e. a short distance at their opposite portions, the current flow concentrates there and the active layer of the LED chip has its electric current unevenly flowing therethrough, resulting in uneven light emission.
Furthermore, where providing a current diffusion layer through which the light is radiated, the electric resistance of the current diffusion layer cannot be reduced to a sufficient extent when the light is to be transmitted therethrough. Conversely, if the electric resistance of the same layer is to be lowered, the light is shielded resulting in lowering in the light radiating efficiency, i.e. the rate of the light that can be radiated outward of the device. On the other hand, it is impossible to increase the area of the p-side electrode 28 to a sufficient extent because the p-side electrode 28 shields the light completely. Meanwhile, light emission is done by carrier recombination within the active layer as stated before, so that it is preferred that the electric current flows spreading through the active layer as broad as possible at its plane. To this end, there is a problem that the current diffusion layer 27 and the electrodes 28, 29 have to be formed in a manner satisfying reciprocal functions of light shield and current diffusion.